Vibrating sensors, such as for example, vibrating densitometers and Coriolis flowmeters are generally known, and are used to measure mass flow and other information for materials flowing through a conduit in the flowmeter. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. Nos. 4,109,524, 4,491,025, and Re. 31,450, all to J. E. Smith et al. These flowmeters have one or more conduits of a straight or curved configuration. Each conduit configuration in a Coriolis mass flowmeter, for example, has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the conduit(s), and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or a small “zero offset”, which is a time delay measured at zero flow. As material begins to flow through the flowmeter, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the flowmeter lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pickoffs on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pickoffs are processed to determine the time delay between the pickoffs. The time delay between the two or more pickoffs is proportional to the mass flow rate of material flowing through the conduit(s).
Material flow through a flow tube creates only a slight phase difference on the order of several degrees between the inlet and outlet ends of an oscillating flow tube. When expressed in terms of a time difference measurement, the phase difference induced by material flow is on the order of tens of microseconds down to nanoseconds. Typically, a commercial flow rate measurement should have an error of less than 0.1%. Therefore, a Coriolis flowmeter must be uniquely designed to accurately measure these slight phase differences.
It is a particular problem to measure minimal flow rates of materials flowing through a pipeline. It is, however, also known to use a single loop, serial path flow tube to measure relatively low rates of fluid flowing through a pipeline. A flowmeter measuring small flow rates must be formed of relatively small components including tubes and manifolds. These relatively small components present a variety of challenges in the manufacturing process including, but not limited to, difficult welding or brazing processes. First, it is difficult to weld thin-walled tubing. Second, the welds and joints generally do not provide the smooth surface needed for sanitary applications of the flowmeter, as such applications demand a continuous, smooth flow tube surface that does not promote adhesion of material to the walls of the flow tube.
In order to employ a continuous tube surface suitable for low flow rates, a dual loop, single tube sensor may be employed, wherein the flow tube is brazed to an anchor block that supports the flow tube within the flowmeter. As part of the assembly process, the flow tube is completely restrained by being brazed to the anchor block. Unfortunately, as the flow tube and anchor block cool, they do so at different rates, which causes large stresses at the tube-to-anchor braze joints, which can lead to cracks at the braze joints. In dual tube sensors, the set of flow tubes may freely expand and contract as part of the brazing temperature cycle, thereby reducing the residual stress developed in the braze joint.
Therefore, there is a need in the art for an apparatus and method to allow the brazing of anchor blocks to a multi-loop, single flow tube sensor that may accommodate expansion/contraction cycles from heating. The present invention overcomes this and other problems, and an advance in the art is achieved.